Weed life cycles

Vegetative reproduction

Production of new plants from vegetative structures

Clones=daughter plants=ramets

Genetically identical to the parent plant

Sexual reproduction

Production of new plants from seeds

Allows for genetic mixing/diversity in offspring (genets)

Types of reproduction in plants

Summer annual

Germinate in spring and summer and mature before winter

Problems in spring and early summer planted crops, open sites, winter cereal crops, edges of natural areas

Lambsquarter, redroot pigweed, bedstraw, wild buckwheat, green foxtail, wild oat

Winter annual

Germinate in the late summer, fall, and winter (in warmer regions)

Mature in spring or early summer

Problems in late summer and fall planted crops, open sites, bare spots in pastures

In colder regions, winter annuals may be summer annuals

Shepherd’s purse, field pennycress, downy brome

Five life cycle categories for weeds

Biennial

Live for two growing seasons

Seeds germinate and plants grow vegetatively as rosettes the first year

Exposure to cold in winter causes the rosette to bolt in the spring

The biennial then flowers, matures its seed, and dies during the summer or fall of the second growing season

Usually a problem in perennial fields; not in fields that are disturbed in fall

Henbit, houndstongue

Perennial

Survive indefinitely (3+ years)

Flowering doesn’t trigger senescence

Underground structures continue to live

Five life cycle categories for weeds

Simple perennials Start from seed, grow vegetatively

Form a crown of tissue at or below the soil surface on the upper end of a taproot

Root and crown survive indefinitely

Shoots periodically produce flowers and seeds

What is the purpose of seeds?

Found in perennial crops, undisturbed areas, no-till fields

Spotted knapweed

Creeping perennials Reproduce from vegetative structures and seed

Vegetative reproductive structures are the major means of localized spread, competition, and survival

Creeping roots, rhizomes, stolons, bulbs, tubers

Have large amounts of stored food and numerous buds

Field bindweed, quackgrass, Canada thistle

Five life cycle categories for weeds

Vegetative reproductive structures

Rhizomes

Elongated horizontal underground stems with nodes, internodes, and modified leaves

Root and new shoot growth always originates from buds at the nodes

Tubers

Thickened underground stems borne on rhizomes

Internodes are shorter than those on a rhizome

Root and new shoot growth always originates from buds at the nodes

Vegetative reproductive structures

Bulbs

Modified leaf tissue borne on a small stem plate

Roots and new bulbs develop from this stem plate

Stolons

Horizontal aboveground stems

Leaves produced at the nodes are green

Food storage not a major function

Vegetative reproductive structures

Creeping roots

No leaves, nodes, or internodes

Can grow downward and horizontally

Can produce shoots along their length

Tend to penetrate deeper than other vegetative reproductive structures and are harder to control

Shoot Regeneration in Weed Species

Shoot regeneration

All plants have a growing point from which they can regenerate shoots

Any practice that destroys the plant below the lowest growing point will kill the plant

The position of shoot regeneration on a plant and the resistance of a plant to destruction depends on the life cycle

Annuals

Broadleaves

The bud in the axil of the cotyledons in the lowest point from which an annual broadleaf can regenerate a new shoot

Any action that destroys the weed below that point should kill it

Grasses

The growing point, or crown, is just below the soil surface

Annual grasses need to be damaged under the soil surface for complete kill

Shoot regeneration

Biennials Rosette stage difficult to control

As the seed stalk begins to elongate (bolt) the meristems raise aboveground

Simple Perennials Crown is 1 to several inches below soil surface

Crown and taproot survive and generate new shoots even after substantial damage

How would you go about trying to control a simple perennial (mechanically?)

Creeping Perennials Numerous well-protected buds capable of generating new shoots

Large amounts of stored food and many buds

Usually 6 in belowground, sometimes several feet

Severe disturbance usually helps them regenerate

Need repeated action to deplete the plant of energy reserves

Questions!

1. Summer and winter annuals germinate in spring and summer and mature before winter. True or False?

2. _______________ weeds are most troublesome in perennial and no-till cropping systems.

a. Perennial

b. Annual

c. Biennial

d. Centennial

3. What stage of a biennial weed is most difficult to control? Why?

Field pennycress Scientific name: Thlaspi arvense

Family: Brassicaceae

Life cycle: Annual

Where found: Cropland

Physical description:

Seed: fingerprint pattern

Cotyledon: oval with long petiole

True leaves round to spatulate, then oblong to oval

Forms a basal rosette at first, then alternate when flowering

Early leaves have long petioles; later stem leaves are sessile, clasping around stem

Leaves without hair, in contrast to shepherd’s purse

Growth habit: erect

Interesting facts:

Seed pods are disc-shaped with a distinct notch at tip

Distinct, garlic-like odor when brushed

Latin name describes flat pod

Thlas=to crush or flatten

Shepherd’s Purse Scientific name: Capsella bursa-pastoris

Family: Brassicaceae

Life cycle: Annual

Where found: Cropland

Physical description: highly variable

Seed: small, sticky when wet

Cotyledon: ovate, apex may be indented

Growth habit: basal rosette; flowering stem alternate

True leaves oval, then elliptic to oblanceolate, most becoming pinnately divided

Stem leaves lanceolate

Early leaves have long petioles; later stem leaves are sessile, clasping around stem

Leaves are sparsely hairy

Interesting facts:

Seed pods heart-shaped, flattened

Latin name describes seed pods

caps=small box

bursa= purse, pastor=shepherd